1. Field of the Invention
The present invention relates to improvements in the construction and manufacture of polymeric film and bags. In particular, the present invention relates to improvements in varying the thickness of film formed from a blown film extrusion process and constructing polymeric bags from the resultant film.
2. Description of the Related Art
Polymeric bags are ubiquitous in modern society and are available in countless combinations of varying capacities, thicknesses, dimensions, and colors. The bags are available for numerous applications including typical consumer applications such as long-term storage, food storage, and trash collection. Like many other consumer products, increased demand and new technology have driven innovations in polymeric bags improving the utility and performance of such bags. The present invention is an innovation of particular relevance to polymeric bags used for trash collection.
Polymeric bags are manufactured from polymeric film produced using one of several manufacturing techniques well-known in the art. The two most common methods for manufacture of polymeric films are blown-film extrusion and cast-film extrusion. In blown-film extrusion, the resulting film is tubular while cast-film extrusion produces a generally planar film. The present invention is generally applicable to drawstring trash bags manufactured from a blown-film extrusion process resulting in tubular film stock. Manufacturing methods for the production of drawstring bags from a web of material are shown in numerous prior art references including, but not limited to, U.S. Pat. Nos. 3,196,757 and 4,624,654, which are hereby incorporated by reference.
In blown film extrusion, polymeric resin is fed into an extruder where an extrusion screw pushes the resin through the extruder. The extrusion screw compresses the resin, heating the resin into a molten state under high pressure. The molten, pressurized resin is fed through a blown film extrusion die having an annular opening. As the molten material is pushed into and through the extrusion die, a polymeric film tube emerges from the outlet of the extrusion die.
The polymeric film tube is blown or expanded to a larger diameter by providing a volume of air within the interior of the polymeric film tube. The combination of the volume of air and the polymeric film tube is commonly referred to as a bubble between the extrusion die and a set of nip rollers. As the polymeric film tube cools travelling upward toward the nip rollers, the polymeric film tube solidifies from a molten state to a solid state after it expands to its final diameter and thickness. Once the polymeric film tube is completely solidified, it passes through the set of nip rollers and is collapsed into a collapsed polymeric tube, also referred to as a collapsed bubble.
One common household item formed from the polymeric film produced by a blown film extrusion process are drawstring trash bags. Frequently, drawstring trash bags are used to line a rigid container, such as a trash can, for the disposal of household trash. When used with a rigid container, it is common to fold the upper opening of a drawstring trash bag over the upper rim of the container to keep the upper opening of the bag accessible. Some rigid containers provide retaining devices to hold the upper opening of a drawstring bag in place while, in other instances, the drawstring bag may provide certain features or properties that facilitate keeping the bag in place on the container. The use of these rigid containers is particularly common in connection with 13-gallon drawstring bags. Rigid containers may also be used with larger bags, such as those commonly used for the collection of outdoor or yard waste. When drawstring bags are used with supporting rigid containers, the drawstring trash bag is often filled with trash and other debris until the capacity of the container or trash bag is reached.
After filling the drawstring bag with trash or debris, the bag and its contents are generally pulled out of the rigid container in one of several ways. Some consumers may prefer to pull the drawstrings taut, using the drawstrings to pull the bag and its contents out of the container. However, in many cases, consumers pull the bag out of the rigid container before pulling the drawstrings taut. In such cases, a consumer will typically grab the upper opening of the bag hem or, more commonly, the area immediately below the hem. As the consumer grabs the bag and pulls it out of the container, significant force can result in the area immediately below the hem. This force on the hem is due to the weight of the bag contents and the pulling action by the consumer, which can cause the polymeric film to puncture or fail in this region of the bag—especially when the contents of the bag are particularly heavy. Thus, there is a need to improve the strength and puncture resistance of the hem and the area immediately below the hem.
Another aspect of prior art drawstring bags that may be subject to improvement is the tendency for the hems of drawstring bags to detach if there are even minor manufacturing defects. In particular, if the pressure or heat used to create the hem seal across the width of the drawstring trash bag is too great, the integrity of the drawstring trash bag may be endangered due to crystallization of the polymeric film along the hem seal. In these cases, when the drawstring trash bag is filled with trash or debris, the hem seal may fail causing the entire length of the hem, and the drawstring contained within the hem, to “zipper” or detach from the rest of the bag. Thus, there is a need to improve the robustness of the hem seal to ensure that the drawstring is retained about the opening of the trash bag.
Another further improvement of prior art drawstring trash bags may be to reduce the cost of producing the trash bags. A substantial cost of manufacturing trash bags can be related to the cost of polymeric resin used to produce a bag. One way to decrease the cost of producing bags is to decrease the thickness of film used in their manufacture, which reduces the amount of polymeric resin required to make a bag. However, reducing the thickness of film used to produce a bag can lead to a reduction in the strength of the bag. Thus, a need exists to decrease the use of resin to produce drawstring trash bags while maintaining enough strength so that the bags do not fail when used.
One method of thinning out the material of a bag is by incrementally stretching the film. Stretching the film incrementally allows stretching the film in only selected areas of the film. Incrementally stretching also provides for a ribbed pattern to be placed on the bag that may serve to differentiate the appearance of the bag. The incremental stretching can maintain the thickness of the bag in segments that are not stretched that can be referred to as thick ribs. These thick ribs may be perceived by an end user of the bag such that the bag is perceived to be as thick as the previously un-stretched film.
In the prior art, U.S. Pat. No. 4,116,892 to Schwarz discloses using a pair of grooved intermeshing rollers to incrementally stretch an orientable polymeric material in a direction transverse to the machine direction. In a related field, U.S. Pat. No. 5,041,255 to Zafiroglu discloses the use of a pair of intermeshing circumferentially ribbed rollers for stretching portions of a fibrous web.
In a much later publication, US Pat. Appl. Pub. 2012/0088645A1 discloses use of intermeshing rollers, each roller having a plurality of circular ridges protruding outwards, to stretch a portion of a polymeric web in a direction transverse to the machine direction. The application further discloses that the use of the aforementioned intermeshing rollers can be used on the film so that the panels of the trash bag are imparted with a ribbed pattern running in a lengthwise direction, parallel to the direction that the film is extruded.
In light of the foregoing, it would be desirable to provide a means for reinforcing the drawstring trash bag in the area immediately adjacent/below the hem area of the trash bag while also controlling the thickness of the reinforced areas. It would also be desirable to provide additional improvements to the manufacture of drawstrings bags by thinning out the bags in desirable locations to provide targeted strengthening and light weighting of the bag. The present invention represents a novel solution to address this needs.